1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to a color image forming apparatus with a function for correcting alignment errors in the positions of monochromatic images, i.e., color registration errors among the monochromatic images.
2. Description of the Related Art
As a background technology related to the present invention, conventional image forming apparatuses are known which form a multicolored image by superimposing two or more monochromatic images on a record medium after correcting color registration errors of among the monochromatic images. Such image forming apparatuses include image forming means for separately forming monochromatic images on an image carrier, measurement means for measuring the monochromatic images formed on the image carrier, abnormal position storing means for storing positions of the monochromatic images whose measurement information measured by the measurement means is abnormal, and correcting means for correcting the color registration errors based upon the measurement information of the monochromatic images which rest on positions except for the positions stored in the abnormal position storing means (see, for example, Japanese Unexamined Patent Publication No. 2004-294471).
In order to maintain a good property of an image in the conventional color-image forming apparatus, it is necessary to print test patterns regularly and detect a position of each printed test pattern to correct alignment errors in the relative positions at which monochromatic images are formed, that is, correct color registration errors among the monochromatic images. However, it has taken much time to correct such errors, which restrain the apparatus from forming the images. Here, a new technology will be needed to correct such errors during a shorter time period. Further, a toner is consumed for preparing the test patterns. In particular, when rotation phase errors among a plurality of photosensitive drums are corrected, a great number of test patterns are needed to detect the rotation phase. Therefore, there will be a demand for a technology to correct the errors efficiently using a smaller number of test patterns.